1. Field of the Invention
The present invention relates to an active matrix display device among electronic circuits constituted by using crystalline thin film semiconductors.
2. Description of the Related Art
Since a technique of manufacturing a thin film transistor (hereinafter referred to as TFT) by using an amorphous or crystalline semiconductor film formed on a glass substrate or a quartz substrate has been known, an attempt of applying this technique to an active matrix display circuit has been made. The simplest structure is such that only an active matrix circuit is constituted by the TFTs, and circuits for driving the active matrix circuit, such as a data driver (source driver) and a scan driver (gate driver), are constituted by integrated circuits using single crystal semiconductor.
However, this method requires a technique for connecting a large number of terminals between the active matrix circuit and the respective driver circuits, so that it is disadvantageous in enhancing the integration. On the other hand, there is a proposal in which a driver circuit is also constituted by TFTs in addition to an active matrix circuit (Japanese Patent Publication No. Hei 5-9794 and Japanese Patent Publication No. Hei 2-61032, etc.).
Like the above described structure, an active matrix display device in which a driver circuit and an active matrix circuit are formed on the same substrate is called a monolithic type active matrix display device. When the active matrix display device is the monolithic type, wiring lines required to be connected from the outside of the substrate are limited to those for only power supply, video signals, and synchronous signals, so that the monolithic type is advantageous in integration.
For driving a driver circuit, when using a silicon film as an active layer, a TFT is limited to those which include an active layer of crystalline silicon (polysilicon), and such a TFT is referred to as a high temperature polysilicon TFT or a low temperature polysilicon TFT according to process temperatures of a silicon film.
The high temperature polysilicon TFT is formed by a technique using a heat treatment at a relatively high temperature such as 800xc2x0 C., 900xc2x0 C. or more, as means for forming a crystalline silicon film. This technique may be called a derivative technique of IC manufacturing processes using a single crystal silicon wafer. As a substrate on which the high temperature polysilicon TFT is formed, a quartz substrate capable of withstanding the temperature at the heat treatment is naturally used.
On the other hand, the low temperature polysilicon TFT is formed by using an inexpensive glass substrate (its heat-resisting property is naturally inferior to the quartz substrate) as a substrate. In the production of a crystalline silicon film constituting the low temperature polysilicon TFT, there is used a heat treatment at a temperature of not higher than 600xc2x0 C. against which the glass substrate is able to withstand, or a laser annealing technique which hardly gives thermal damage to the glass substrate.
The technique of manufacturing the high temperature polysilicon TFT has a feature that TFTs having uniform characteristics can be integrated on the substrate. On the other hand, the technique of manufacturing the low temperature polysilicon TFT has a feature that the glass substrate, which is inexpensive and is easily formed to have a large area, can be used as the substrate.
Incidentally, in the technique under the present circumstances, there is no large difference in characteristics between the high temperature polysilicon TFT and the low temperature polysilicon TFT. As a subtle difference, the high temperature polysilicon is superior in uniformity of the production yield and the characteristics in the surface of the substrate, and the low temperature polysilicon is superior in the productivity and the production cost.
In both of the high temperature and low temperature polysilicon TFTs, there have been obtained such characteristics that the mobility is about 50 to 100 (cm2/Vs), and S-value is about 200 to 400 (mV/dec)(VD=IV). The characteristics are such that it is possible to realize a high speed operation higher than a TFT using amorphous silicon by a factor of about double figures. However, the characteristics are largely inferior to those of a MOS transistor using a single crystal silicon wafer. In general, the S-value of the MOS transistor using the single crystal silicon wafer is about 60 to 70 (mV/dec), and the operation frequency thereof is higher than that of the high temperature polysilicon TFT or the low temperature polysilicon TFT by a factor of about single figure to double figures.
Since a data driver circuit using the high temperature or low temperature polysilicon TFT having such characteristics has the limit in signal processing capacity, it is necessary to make a specific design for constituting a large scale matrix. For example, if the matrix is a small scale matrix such that the number of pixels (the number of pixel electrodes of an active matrix circuit) is less than fifty thousands, the basic structure shown in FIGS. 1A and 1B is sufficient.
FIG. 1A shows an active matrix circuit 3, and a scan driver 2 and a data driver 1 for driving the active matrix circuit 3. The active matrix circuit 3 is connected to the scan driver 2 and the data driver 1 by a large number of wiring lines 5 and 4. Since these wiring lines are formed at the same time when the above circuits are formed, there is no difficulty in the production. A large number of pixels 6 are disposed in the active matrix circuit 3, and each of the pixels includes a switching transistor 7 and a pixel electrode 8. A plurality of switching transistors may be used (FIG. 1A).
FIG. 1B shows the details of the data driver circuit. That is, the data driver circuit has such a structure that in accordance with pulses sequentially generated from a shift register, a video signal is sampled by sampling transistors, and the signals are stored by analog memories (capacitors), and when sampling for all rows is ended, analog switches (and analog buffers) are concurrently driven by a latch pulse, and the signals are sent to the active matrix (FIG. 1B).
For example, if the number of pixels are less than fifty thousands, in order to process the picture image information of thirty frames per one second, the processing speed of the data driver is sufficient when it is fifty thousands (pixels)xc3x97thirty (frames/second )=1.5 MHz.
This is a speed which can be handled by the conventional high temperature or low temperature polysilicon TFT. However, if the number of pixels is increased, TFTs cannot follow operation speed. A first method of solving this problem is to provide plural lines of shift registers. For example, two lines of shift registers are provided in parallel to each other, and the respective registers are made to transmit pulses the phases of which are shifted by half a period.
A second method is to provide plural lines of video signals. For example, four lines of video signals are provided, and these are sampled by one shift register, so that an operation speed can be reduced to xc2xc. An example will be explained with reference to FIG. 9. When a pulse is generated from a shift register of n-th stage, sampling is carried out by four sampling transistors connected to respective signal lines of video signals 1 to 4. The subsequent operation is the same as the case of FIG. 1B. In this way, since one stage of shift register can drive four columns of data lines, when the number of data lines is 4N, it is sufficient that the number of stages of the shift register is N. Thus, as compared with the case of FIG. 1B, the operation speed can be reduced to xc2xc (FIG. 9).
In order to adopt such a system, it is necessary to divide the video signal to xc2xc. FIG. 10 shows such a circuit which is constituted by four stages of shift registers 1 to 4. A sampling transistor and an analog memory similar to the data driver circuit are disposed at the output of the respective shift register (FIG. 10).
At the timing when a pulse is generated from the shift register of each stage to the sampling transistor, sampling of the video signal is sequentially performed, and this sampled signal is stored in the analog memory. At the timing when the fourth sampling transistor briefly operates, all the four analog switches operate so that video signals 1 to 4 are outputted.
Of course, the operation of a one-fourth frequency division circuit is high, and this circuit can not be disposed on the same substrate as the active matrix circuit. Thus, as shown in FIG. 11, this circuit is formed at the outside of the substrate by using a single crystal semiconductor. Further, four vide o signal lines and a synchronous signal line (clock signal line)and the like are needed to be connected to the active matrix display device (FIG. 11).
It is often carried out that an operation speed is further decreased by combining the first method and the second method described above. For example, in a display device of VGA specification, there are 640 linesxc3x97480 linesxc3x97three original colors=921600 pixels. In order to drive the pixels in 30 frames/set, a high speed operation of 28 MHz is required. However, such a high speed operation can not be achieved by well known high temperature or low temperature polysilicon TFTs.
However, for example, as shown in FIG. 12, when a screen is divided into upper and lower screens, four-divided video signals are inputted into the respective data drivers provided for the upper an d lower screens, and two lines of shift registers are provided for the respective data drivers, the operation speed can be decreased to {fraction (1/16)}, that is, 1.7 MHz. However, a circuit for dividing the video signal to xc2xc, and a circuit for generating pulses inputted into the shift register, are required to have operation performance of 28 MHz. These circuits can not be realized by the TFTs, so that they are externally provided. Thus, at least eight video signal lines and two synchronous signal lines for supplying pulses to the respective shift registers are required (FIG. 12).
In addition, there is a problem that due to a minute discrepancy of timing of division or the like, stripe patterns appear on a screen. Further, it is conceived that an oscillation circuit, a D/A converter, an A/D converter, and a digital circuit for performing various kinds of picture image processing, other than the driver circuit, are integrated on the same substrate (for example, Japanese Patent Unexamined Publication No. Hei 7-135323). However, the above oscillation circuit, D/A converter, A/D converter, and digital circuit for performing various kinds of image processing, are needed to operate at a frequency further higher than that of the driver circuit. It is practically impossible to constitute those circuits by the high temperature polysilicon TFT or low temperature polysilicon TFT obtained by the technique under the present circumstances.
An object of the present invention is therefore to provide an active matrix display device by using thin film transistors capable of constituting circuits required to realize the above-mentioned high speed operation (in general, an operation speed of more than several tens IMHz).
The present invention is characterized by comprising the following two structures in a monolithic active matrix display device using TFTs formed over a substrate having an insulating surface.
As the substrate having the insulating surface, a glass substrate (although it is required to have heat resistance against a process temperature, e.g. an alumina glass substrate), a quartz substrate, and a semiconductor substrate on the surface of which an insulating film is formed, are exemplified.
The first structure is that when the number of lines of shift register circuits constituting a data driver circuit is p, the number of video terminals inputted to the data driver circuit from the outside of the substrate is q, and the number of pixel electrodes existing in an active matrix circuit and driven by the data driver circuit is R, R/pq is from fifty thousands to three millions.
The second structure is that an active layer of a thin film transistor constituting the data driver circuit is a semiconductor film crystallized by a heat treatment under the existence of a catalytic element for promoting the crystallization.
As the catalytic element for promoting the crystallization of semiconductor, nickel is extremely preferable from the viewpoint of reproducibility and effect. As other catalytic elements, one kind or plural kinds of elements selected from the group consisting of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au may be used.
Methods of introducing a metal element include a method of coating a solution containing the metal element, a method of CVD, a method of sputtering or evaporation, a method of plasma treatment using an electrode containing the metal element, and a method of gas adsorption. Those methods are disclosed in, for example, Japanese Patent Unexamined Publication No. Hei 6-244104 (sputtering method), Hei 7-130652 (solution coating method), and Hei 7-335548 (CVD method).
The catalytic element may be introduced into the entire surface of a semiconductor film, or may be selectively introduced. If the latter method is adopted, the direction of crystal growth can be controlled. However, for the selective introduction, a step of forming a mask is required.
The temperature of heat treatment for crystallization is 450 to 750xc2x0 C., preferably 550 to 650xc2x0 C. An atmosphere at the crystallization is an inert gas atmosphere including oxygen as little as possible, such as a nitrogen atmosphere.
A manufacturing method of a crystalline semiconductor film constituting the active layer of the present invention is characterized by including a step of gettering the catalytic element in the crystalline semiconductor film by carrying out a second heat treatment so as to decrease the catalytic element in the crystalline semiconductor film after a step of crystallization by a first heating treatment, in addition to the above-mentioned two structures.
One means of gettering treatment is a method of performing gettering of the catalytic element in the crystallized semiconductor film by the operation of a halogen element through a heat treatment in an atmosphere including the halogen element. Other means is a method in which impurities in group 15 or in group 15 and group 13 are selectively added into the crystallized semiconductor film and a heat treatment is carried out, so that gettering of the catalytic element is carried out in the region where the impurities were added.
In the case where a gettering step of using a halogen element is adopted, it is preferable to carry out the above heat treatment at a temperature exceeding 700xc2x0 C. At a temperature lower than 700xc2x0 C., it becomes difficult to decompose a halogen compound in the processing atmosphere, so that there arises a fear that the gettering effect can not be obtained. Thus, the temperature of the heat treatment is made preferably 800 to 1000xc2x0 C. (typically 950xc2x0 C.), and a time of the treatment is made 0.1 to 6 hours, typically 0.5 to. 1 hour.
A typical gettering step is a step of heat treatment at a temperature of 950xc2x0 C. and for 30 minutes in an oxygen atmosphere containing hydrogen chloride (HCI)of 0.5 to 10 vol % (3 vol % in an embodiment). If the concentration of HCL is more than the above, roughness almost equal to a film thickness is formed on the surface of the crystalline semiconductor film so that it is not preferable. Further, it is also effective to introduce hydrogen gas into the atmosphere and to use the reaction of wet oxidation. As gases for adding a halogen element into an atmosphere, other than the HCI gas, one kind or plural kinds of gases selected from the group consisting of compound gases including the halogen element and halogen gases, such as HF, NF3, HBr, C12, ClF3, BC13, F2 and Br2, may be used.
In the case where a nickel element is used as the catalytic element, the concentration of nickel finally remaining in the semiconductor film is about 1xc3x971014 atoms/cm3 to 5xc3x971018 atoms/cm3. The measurement of this concentration may be carried out by using SIMS (secondary ion measurement system).
When the atmosphere of gettering is made an oxidizing atmosphere such as oxygen or water vapor, the effect of gettering is promoted. When the atmosphere is made the oxidizing atmosphere, a thermal oxidation film is formed on the surface of the crystalline semiconductor thin film, and the catalytic element is concentrated in the thermal oxidation film. If the gettering condition of the thermal oxidation film is adjusted, the upper limit of this concentration can be decreased to about 5xc3x971017 atoms/cm3. If the thickness of the thermal oxidation film is thicker than the semiconductor thin film, a TFT having superior characteristics can be obtained. Since the catalytic element in the semiconductor film is concentrated in the thermal oxidation film, the thermal oxidation film is preferably removed after the gettering step.
In order to improve the characteristics of the semiconductor device, after the thermal oxidation film was once removed, a heat treatment is again carried out under the same condition as the second heat treatment, so that a thermal oxidation film may be formed on the surface of the semiconductor film. It is needless to say that it is preferable to make the thickness of the thermal oxidation film obtained at that time thicker than the semiconductor film.
Since the catalytic metal element is converted into a halogen compound and is vaporized into the atmosphere by the heat treatment in the halogen atmosphere, a gradient or distribution of the nickel concentration occurs in the thickness direction of the obtained crystalline semiconductor film. In general, it is observed that the concentration of the metal element in the crystalline semiconductor film has a tendency that the concentration of the metal element increases toward the interface where the thermal oxidation film is formed. According to a condition, it is observed that the concentration of the metal element tends to increase toward the substrate or an under film, that is, toward the interface at the back side. The halogen element also has the concentration distribution similar to that of the metal element. That is, it exhibits the concentration distribution in which the concentration increases toward the top side surface and/or back side surface of the crystalline semiconductor film.
In the case where impurities in group 15 or in group 15 and group 13 are used as means of gettering, it is most suitable to use phosphorus among the impurity elements in group 15. As the impurities in group 13, boron is the most suitable, and antimony is suitable next to boron.
The heat temperature in this case is 400 to 1050xc2x0 C., and preferably 600 to 650xc2x0 C. By this heat treatment, the gettering of the catalytic element is performed in the region where the impurities in group 15 or in group 15 and group 13 has been added, and the concentration of catalytic element in other regions is lowered to less than 5xc3x971017 atoms/cm3.
Through the above two gettering treatments, the lower limit of nickel concentration in the crystalline semiconductor film is generally about 1xc3x971016 atoms/cm3. This is because in view of the cost, it is normally difficult to cancel the influence of nickel element attached to the substrate or the device so that such an amount of nickel element remains. That is, in the case where conventional manufacturing steps are adopted, the concentration of remaining nickel element is about 1xc3x971016 atoms/cm3 to 5xc3x971017 atoms/cm3. However, it is possible to decrease the concentration of the remaining element by optimizing the degree of cleansing of the device or manufacturing steps.
A gate insulating film of a thin film transistor of the data driver circuit of the present invention is characterized by including a thermal oxidation film formed by thermal oxidation of an active layer. The forming temperature of this thermal oxidation film is extremely important. In order to obtain such a TFT as is capable of making a single device operate at a speed of more than several tens MHz as described later and has an S-value of not larger than 100 (mV/dec), the heating temperature at the formation of the thermal oxidation film is needed to be preferably made 800xc2x0 C. or more, and more preferably 900xc2x0 C. or more. On the other hand, it is suitable that the upper limit of the heat temperature is made about 1100xc2x0 C. which is the upper limit of heat-resisting temperature of the quartz substrate.
The final film thickness of the crystalline semiconductor film of the present invention is preferably 100 xc3x85 to 750 xc3x85, more preferably 150 xc3x85 to 450 xc3x85. By realizing such a film thickness, the crystalline structure as shown in FIGS. 6 to 8 can be obtained more prominently and with good reproducibility.
The final film thickness of the crystalline semiconductor film is needed to be determined by taking the decrease of film thickness due to film growth of the thermal oxidation film into consideration.
By adopting the above steps, the crystalline semiconductor film of the present invention can be obtained. Further, it is possible to obtain a TFT using the uniqueness of the crystalline structure. The thus obtained TFT can realize the above-mentioned first structure of the present invention. In the display device of the present invention, of course, as shown in FIGS. 11 and 12, division (frequency division) of a video signal or multiplication of lines of shift registers may be conducted. However, when considering that the present invention is applied to the active matrix display device of the most simple structure as shown in FIG. 1, it is desired that p=q=1 in the first structure.